It comes ahead of planned protests that are due to be held on Tuesday.

Fars quoted an unnamed official in Iran's judiciary as denying that the men had been taken.

"The two are currently in their homes and there have only been restrictions on their contacts," the official said.

'Security vans'

Both Mr Mousavi and Mr Karroubi have called for demonstrations in Iran in the light of the recent uprisings in Tunisia and in Egypt.

Earlier this month the two men, along with their wives, were detained in their respective homes in Tehran as protests were staged on the streets of the capital.

On Monday one of Mr Karroubi's sons told BBC Persian service he had been told his father had been "taken by security forces to an unspecified location".

Mr Mousavi's Kaleme website reported that the men and their wives "have been arrested and were transferred to the Heshmatiyeh prison of Tehran".

"According to credible sources, the arrest and the transfer to jail is sure but the date when it occurred is still uncertain," it said.

A neighbour said that he had seen eight security vans outside Mr Karroubi's home late on Thursday night. Shortly afterwards the vans then left the area along with one car, he said.

Earlier on Monday, Iran's state prosecutor Gholam Hossein Mohseni Ejehi was quoted by the official Irna news agency saying that the country's authorities have restricted the two men's meetings and phone calls.

"Should circumstances arise, other measures will be taken," he added.

Thousands of supporters of Mr Mousavi and Mr Karroubi took to the streets of Tehran on 14 February, amid clashes with security forces which left two dead.

Some 1,500 people were detained, opposition groups said, but official figures put the number at 150.

Following the clashes, Iranian MPs called for the two opposition leaders to be tried and executed.